Mass filter having an ion source structure with preselected relative potentials applied thereto



Nov. 8, 1966 MASS FILTER HAVING Filed Feb. 4, 1963 HF GENfiRATOR KARL-GEORG GUNTHER 3,

AN ION SOURCE STRUCT RELATIVE POTENTIALS APPLIE D THERETO URE WITH PRESELECTED 2 Sheets-Sheet 1 RECORDER N 8, 1966 KARL-GEORG GUNTHER 3, 6

MASS FILTER HAVING AN ION SOURCE STRUCTURE WITH PRESELECTED RELATIVE POTENTIALS APPLIED THERETO 2 Sheets-Sheet 7:

Filed Feb. 4, 1963 FIG. 5

FIG. 3

United States Patent Ofiice 3,284,628 Patented Nov. 8, 1966 My invention relates to ion separating methods and apparatus and particularly to those using the type of mass filter or spectrometer disclosed in Patent No. 2,939,952 of Paul et al., or its equivalent.

The Paul et al. method utilizes an electric field which is periodical in time and wherein the potential of the electric field is a quadratic function of the coordinates x, y, z. The most general such potential is defined by the equation:

( p in z )=f( +B -/z Here f(t) is an arbitrary periodic function of the time t. The constant a, B, and 'y satisfy the equation ot+fi='y. Ions travel through such a field in two different kinds of ion paths; either the ions perform oscillations around the center of symmetry of the field, the amplitudes of the oscillations remaining smaller than a certain maximum value (stable paths), or the amplitudes of the oscillations increase extremely rapidly so that, within a very short time, the particles impinge on the field generating electrodes and are thus removed (unstable paths). For a particular field and time function f(t), the specific charge, or charge to mass ratio e/ m, of a particular ion will determine whether it travels along a stable or unstable path. If e/m of the ions lies in a stable range, then all its possible paths are stable and, conversely, if e/m of the ion lies in an unstable range, then all its possible paths are unstable. The positions and widths of the stable ranges of the specific charge can be varied within very wide limits solely by varying the amplitude, frequency and/or shape of the field creating voltages which determine the function f(t).

The Paul four-pole mass filter, disclosed in US. Patent No. 2,939,952, has permitted development of practical instruments for measuring partial pressures and having relatively small dimensions. Such instruments, for example, are disclosed in US. Patent 3,075,076 and in pending US. patent applications Serial No. 57,851, filed September 22, 1960 (1 -2084), now Patent No. 3,221,164, Serial No. 94,- 071, filed March 7, 1961 (1 -2158), now Patent No. 3,143,647, and Serial No. 97,244, filed March 21, 1961 (F-2145), now Patent No. 3,105,899, all assigned to the assignee of this application.

The volume of the filter cell (i.e. the analyzer tube) holding the electrodes of a Paul device depends in part on the electrode length. This length afiects the electrical conditions controlling the maximum resolving power and sensitivity of the device. It would seem generally that increased resolving power and sensitivity are incompatible with reduced cell or analyzer volume.

It is an object of my invention to afford decreasing the volume of the measuring cell While nevertheless maintaining the resolving power at a high level or even increasing it.

Another object is to provide an improved method and analyzing apparatus for ion separation, particularly one having increased sensitivity, resolving power, or both.

According to a feature of the present invention, for increasing the sensitivity, the resolving power, or both, of

a particular size Paul four-pole mass filter as described in Patent No. 2,939,952, I apply at the entrance diaphragm of the Paul device a potential which is negative with respect to ground, and I make the potential of the analyzer axis zero or negative. Preferably, I select the respective potentials of the entrance diaphragm and of the axis of the analyzer system to achieve maximum resolving power.

Other objects, advantages, and features of my invention will become obvious from the following description of an embodiment of the invention with reference to the accompanying drawings wherein:

FIG. 1 is a block diagram of a mass filter system for separating ions according to the invention;

FIG. 2 is a schematic representation of the mass-filter cell or analyzer in FIG. 1, together with. an associated D.C. energizing circuit according to the invention;

FIG. 3 is a circuit diagram of a supply system for energizing the analyzer rods according to the present invention;

FIGS. 4a and b, are two voltage graphs showing the respective potentials occurring at the electrode within the device of FIG. 3 for two different sets of applied voltages; and

FIG. 5 is a graph of results obtained with the method of the present invention.

According to FIG. 1, the mass-filter cell 1 is provided with an envelope 5 which contains an ion source 2, a group of rod-shaped deflector electrodes 3 having in dividually a circular cross section. Located at the end of the ion-beam path is a cup-shaped collector electrode 4. The ion source 2 and the collector electrode 4 are coaxially spaced from each other and thus define a center axis 3 for the ion beam issuing from the source 2 toward the electrode 4. The electrode rods 3 are uniformly distributed about the ion-beam axis and extend parallel thereto. A total number of four such electrodes may be used.

The above-mentioned envelope 5 of the cell 1 is vacuum-tightly sealed and has a nipple or neck 6 connected with a tank '7 containing the gaseous mixture to be investigated. The rod electrodes 3 are electrically connected in pairs to a high-frequency generator 3 which supplies electric energy of suitable voltage and frequency. The current due to the ions impinging upon the collector 4 is amplified by an amplifier 9 and supplied to a recorder 10 or other indicating or measuring device. Another measuring instrument 10 is provided for supervising the electron emission of the cathode in the ion source 2.

During operation, a beam of ions is continuously being extracted from the source 2 and is directed toward the collector 4. However, only the ions of a given specific electric charge, or within a given range of charges, can reach the collector 4. Those ions which have different specific charges travel on instable, pendulous paths and thus impinge upon the deflector electrodes 3, thus being filtered out of the mixture. This is more fully explained in the above-mentioned copending applications.

FIG. 2 diagrammatically illustrates the mass-filter cell 1 including the analyzer rods 3, of which there are four as in the Paul patent, and the collector 4. The ion source 2 comprises a heater filament H, anode vessel A and extraction apertures or entrance diaphragms E. A DC. voltage source S energizes the components of the ion source by means of a voltage divider device D. Connected to the device D is the member H at -20 volts to 20 volts, the member A at 50 volts to volts, and the member E at 100 volts to 200 volts, as indicated.

In the diagrammatic illustration of the electric circuit for energizing the analyzer system shown in FIG. 3, the potential of the analyzer rods 3 is applied asymmetrically with respect to ground. High-frequency voltage for the analyzer rods is supplied to the pairs of rods 3 from a high-frequency voltage source 12 through a transformer 13 and blocking capacitors 14. A variable capacitor 17 tunes secondary of transformer 13. Two direct-voltage sources 15 and protective resistors 16 furnish direct voltage to analyzer rods 3. A direct-voltage source 19 is connected between the center point 8 of the secondary of the high-frequency transformer 13 and ground.

FIGS. 4a and b show graphically the potentials prevailing at and bet-ween the components A, H, E, 3 and 4 in FIG. 2 for two values of U The curves have their respective potentials aligned below the components of FIG. 2.

In operation, ionizing electrons are emitted by the heated filament H and are axially shot through an aperture and into the anode vessel A, which receives a positive acceleration potential U Filament H preferably is kept at a potential constituting an optimum value for ionization purposes. The ionizing electrodes serve to ionize particles in the ion source. The positive ions are then drawn from the ion source by a negative extraction voltage, V volts less than the anode voltage U and pass through the rods 3. The axis potential of the rods is maintained at zero or at a negative value, which is more positive by the value U than the extraction voltage. The more positive potential of the rods decelerates the extracted ions so that they will remain or dwell within the analyzer for the required period of time. Preferably, the extraction potential U i.e. the potential dilference between the anode A and apertured extraction electrode E, is sufficiently high to achieve maximum extraction from the ion source 2. The resulting effective ion energy e(U U )=e(U in part determines the ion dwell time and, consequently, the resolving power. The unsymmetrical field effects a higher resolving power than would occur where there is applied a symmetrical collector extraction voltage U E": U U

The resulting empirical relationships are illustrated in FIG. 5. For measurements in argon, the voltage U has been plotted in volts along the abscissa, against values of collector current shown on the left-hand ordinate, and resolving power (m/Am) on the right-hand ordinate. The full-line curve, which is measured in units on the left ordinate, illustrates the intensity curve of the filtered ion current as a function of the counter-voltage U for a constant U The broken-line curve, which is measured in units on the right-hand ordinate, shows the resolving power increasing up to a maximum value and, consequently, shows the efiiciency of the method of the present invention for the same constant U In this experiment, the maximum resolving power (m/Am) max. of approximately 150 occurs at approximately U =-50 v. The experiments were carried out with a device such as illustrated in FIG. 1a.

The occurrence of a maximum in the resolving power is explained as follows. As the value of the effective ion energy e.U *=e(U U decreases, the number of oscillations performed in the high-frequency field increases. This affords a higher resolving power. However, at the same time divergence of the ion beam in the counter-field created by U increases. This again decreases the resolving power. As a consequence, these contradictory effects produce a maximum resolving power at an optimum voltage U The stable mass ion current collected at the collector depends only slightly upon the value of U However, if U approaches U there occurs a decrease in ion cur- .4 rent, and also of the sensitivity of the device, thus also effecting an optimum value for U These experimental results provide a basis for selecting particular values.

The present invention thus affords increasing the sensitivity, the resolving power, or both, in practical fourpole mass filters. Conversely, the invention permits reducing dimensions of the analyzer system, for systems where the requirements of sensitivity and resolving power are the same.

I claim:

1. A system for separating ions having different chargeto-mass ratios, comprising an evacuated vessel, electrode means, means for holding said electrode means in spaced relation in said vessel, ion source means for creating charged particles in said vessel and including in said vessel an anode and an apertured extraction electrode, voltage means for applying to said anode a positive voltage and for applying to said apertured extraction elec trode a negative voltage, and source means for generating a voltage having an arbitrary periodical function f(t) and applying it to said electrode means thereby creating a time-periodical field the potential of which is the general quadratic function of the rectangular coordinates x, y, z of the electrode arrangement, a, {3, and 7 being constants satisfying the equation obi-[3:7, said source means including potential means for superimposing upon the axis of said electrode means a potential having a magnitude in the range from zero to the negative voltage of said apertured extraction electrode.

2. A system for separating ions having different chargeto-mass ratios, comprising an evacuated vessel, electrode means, means for holding said electrode means in spaced relation in said vessel, ion source means for creating charged particles in said vessel and including in said vessel an anode and an apertured extraction electrode, voltage means for applying to said anode a positive voltage and for applying to said apertured extraction electrode a negative voltage, and source means for generating a voltage having an arbitrary periodical function f(t) and applying it to said electrode means thereby creating a time-periodical field the potential of which is the general quadratic function of the rectangular coordinates x, y, z of the electrode arrangement, a, 5, and 7 being constants satisfying the equation u+;8='y, said source means including potential means for superimposing upon the axis of said electrode means a potential having a magnitude in the range from zero to the negative voltage of said apertured extraction electrode, said voltage means and said potential means being adjusted in magnitude values to a determined relationship to each other.

References Cited by the Examiner UNITED STATES PATENTS 2,636,990 4/1953 Gow et al. 250-41.9 2,939,952 6/1960 Paul et al 2504l.9 2,950,389 8/1960 Paul et al 25041.9

RALPH G. NILSON, Primary Examiner.

W. F. LINDQUIST, Assistant Examiner. 

1. A SYSTEM FOR SEPARATING IONS HAVING DIFFERENT CHARGETO-MASS RATIOS, COMPRISING AN EVACUATED VESSEL, ELECTRODE MEANS, MEANS FOR HOLDING SAID ELECTRODE MEANS IN SPACED RELATION IN SAID VESSEL, ION SOURCE MEANS FOR CREATING CHARGED PARTICLES IN SAID VESSEL AND INCLUDING IN SAID VESSEL AND ANODE AND AN APERTURED EXTRACTION ELECTRODE, VOLTAGE MEANS FOR APPLYING TO SAID ANODE A POSITIVE VOLTAGE AND FOR APPLYING TO SAID APERTURED EXTRACTION ELECTRODE A NEGATIVE VOLTAGE, AND SOURCE MEANS FOR GENERATING A VOLTAGE HAVING AN ARBITRARY PERIODICAL FUNCTION F(T) AND APPLYING IT TO SAID ELECTRODE MEANS THEREBY CREATING A TIME-PERIODICAL FIELD THE POTENTIAL OF WHICH IS THE GENERAL QUADRATIC FUNCTION 